This invention relates to coating gas turbine engine airfoils and, more particularly, to a method for masking airfoils during coating to prevent deposition of coating onto selected surfaces of such airfoils.
To provide protection against high temperatures and corrosive effects of hot gases in gas turbine engines, turbine airfoils have been subjected to aluminide coating. Aluminide coating on the flowpath surfaces, platform, angel wing and respective edges provides oxidation and corrosion resistance and enhances the performance life of the airfoil component. It is preferable, however, to avoid coating the dovetail section below the platform of turbine airfoil components because coating can adversely affect fatigue life of the dovetail section. Furthermore, coating the dovetail section is not necessary because in service this section is not in the hot gas flowpath, as is the airfoil section. It is therefore necessary to mask certain sections of the component to prevent coating such sections while allowing coating of other sections.
Two methods for coating turbine airfoils are a codeposition process and a vapor phase aluminide process. These are diffusion coating processes which use aluminum fluoride (AlF3) gas to apply an oxidation-/corrosion-resistant, aluminum-rich layer to the component surface. The codeposition process involves sequential steps of degrease, tape and seal holes, wax, load box, slurry, seal, pack, run, unload and blowoff, water clean, and age. This process utilizes an activated A1-rich powder packed tightly around the airfoil section to facilitate coating while the areas below the platform are packed with an inert powder containing calcined alumina and concentrations of Ni+2 and Cr+3 similar to the parent metal to avoid possible coating, as well as alloy depletion and inter-granular attack. The process yields an aluminide coating that is brittle yet functional for prohibiting corrosion. This process is labor-intensive and operator-dependent, and tends to plug cooling holes and leave excessive amounts of foreign material on the airfoils. It also requires a final heat treatment, without which the coating is too brittle.
The vapor phase aluminide process yields a ductile aluminide coating which requires minimal final heat treatment. The vapor phase aluminide process does not require blow-off or water cleaning operations, thus reducing labor and material costs. One disadvantage of this process, however, is that coating must be removed from the dovetail sections by grinding, and such grinding also removes the beneficial platform edge and angel wing edge coating Thus, there is a need for an effective masking technique that facilitates secure masking of a plurality of turbine airfoil components during vapor phase diffusion coating processes.
Briefly, the invention is directed to a method for simultaneously vapor phase coating airfoil flowpath surfaces and upper platform surfaces of a plurality of turbine airfoils while masking dovetail sections of the turbine airfoils. The method involves inserting dovetail sections of a plurality of turbine airfoils into a dovetail section receptacle which has a chamber with an opening at the top of the receptacle for insertion of multiple turbine airfoil dovetail sections into the chamber, and a rim around the opening, the rim being sized for supporting the platforms of the turbine airfoils, such that the dovetail section of each turbine airfoil extends into the chamber and the platform of each turbine airfoil rests on the rim, with upper platform surfaces and airfoil flowpath surfaces external to the receptacle. The method further involves inserting a spacer adjacent each platform of the turbine airfoils to prevent adjacent platforms from contacting each other, placing the dovetail section receptacle with dovetail sections and spacers therein into a coating apparatus, and introducing coating vapor into the coating apparatus to form a coating on the airfoil flowpath surfaces and upper platform surfaces external to the dovetail receptacle while avoiding coating of the dovetail sections inside the chamber.
The invention is also directed to a dovetail section receptacle apparatus for use in vapor phase diffusion coating of airfoil flowpath surfaces and upper platform surfaces of a plurality of turbine airfoils, while avoiding coating of dovetail sections. The apparatus includes a dovetail section receptacle for receiving dovetail sections of a plurality of turbine airfoils arranged side-by-side. The receptacle has a chamber having an opening at the top of the receptacle for insertion of multiple turbine airfoil dovetail sections into the chamber, and a rim around the opening, the rim being sized for supporting the platforms of the turbine airfoils side-by-side. The apparatus further includes a plurality of spacers for placement adjacent each platform of the turbine airfoils to prevent pairs of adjacent platforms from contacting each other.
In another aspect the invention is directed to a turbine airfoil coating assembly for vapor phase diffusion coating of airfoil flowpath surfaces and platform upper surfaces of turbine airfoils while avoiding coating of dovetail sections thereof. The assembly includes a plurality of turbine airfoils, each having a flowpath surface, a platform having a platform upper surface, and a dovetail section. There is a dovetail section receptacle for receiving dovetail sections of the plurality of turbine airfoils arranged side-by-side, the dovetail section receptacle having a chamber with an opening having a perimeter at the top of the receptacle for insertion of multiple turbine airfoil dovetail sections into the chamber. There is a rim around the opening, the rim being sized for supporting the platforms of the turbine airfoils, such that the dovetail section of each turbine airfoil extends into the chamber and the platform of each turbine airfoil rests on the rim, with upper platform surfaces and flowpath surfaces external to the receptacle. There is a plurality of spacers, with a spacer adjacent each platform of the turbine airfoils to prevent adjacent platforms from contacting each other.